Funds are requested for the purchase of a state of the art circular dichroism spectrophotometer with accessories for stopped flow and low temperature experiments and with a 15 kilogauss electromagnet to enable magnetic circular dichroism to be carried out. The new instrument will replace a thirteen year old model that has become obsolete and unreliable. The new instrument is twenty times more sensitive than the old model and is configured for simultaneous measurement of UV-visible absorption and circular dichroism spectra. The new instrument will also be equipped to make measurements from 175 to 1050 nm, a much expanded spectral window from that currently available. The specific areas of research of the primary users will focus on the use of the new instrument for the following purposes: (a) the continued development of magnetic circular dichroism spectroscopy as a probe of heme iron coordination structure and the initial examination of the use of this technique to study chlorin iron systems, (b) the application of circular dichroism spectroscopy to investigate and quantify the allosteric interconversion of the R and T states of chemically modified hemoglobins, (c) the use of stopped flow circular dichroism spectroscopy to study the mechanism of action of native thymidylate synthase and the use of circular dichroism spectroscopy to characterize selected mutant enzymes generated by site directed mutagenesis, and (d) the investigation of the circular dichroism properties of selenium containing biomolecules, particularly seleno-sulfides and di-selenides, in order to evaluate the potential utility of the method as a probe of the structure of selenobiomolecules.